FIG. 1 is a circuit diagram illustrating a configuration of a multiple output switching power source apparatus according to a related art. In this multiple output switching power source apparatus, an input voltage Vin is a DC voltage produced by rectifying and smoothing an AC voltage of a commercial power source. The input voltage Vin is connected in series with a primary winding P1 of a transformer T1 and a switching element Q1 made of, for example, a MOSFET. A control circuit 11 controls ON/OFF of the switching element Q1.
On the secondary side of the transformer T1, a secondary winding S1 is wound to generate a voltage having a phase opposite to that of a voltage of the primary winding P1 of the transformer T1 and is connected to a rectifying-smoothing circuit. The rectifying-smoothing circuit consists of a diode D1 and a smoothing capacitor C1, to rectify and smooth the voltage induced by the secondary winding S1 of the transformer T1 and provide a first output voltage Vo1 from a first output terminal.
The converter for providing the first output voltage Vo1 is generally known as a flyback converter that accumulates exciting energy in the primary winding P1 of the transformer T1 during an ON period of the switching element Q1, and after the switching element Q2 turns off, discharges the energy through the diode D1 to the output. A feedback circuit 10 feeds back an error signal between the first output voltage Vo1 and a reference voltage to the control circuit 11. According to the error signal, the control circuit 11 adjusts an ON width of the switching element Q1, thereby controlling the first output voltage Vo1 to a predetermined value.
A second output voltage Vo2 and a third output voltage Vo3 are provided by DC/DC converters 21 and 22 that are, for example, step-down choppers connected to the first output terminal of the flyback converter.
This multiple output switching power source apparatus is capable of accurately providing the three output voltages Vo1, Vo2, and Vo3. To provide the second and third output voltages Vo2 and Vo3, however, the DC/DC converters 21 and 22 should have parts such as switching elements, choke coils, and control ICs that increase costs and packaging areas. In addition, the switching elements must turn on/off paths for passing a large current, to cause excessive switching loss and noise. If the outputs must be insulated from one another to prevent interference among circuits, the circuit configuration of the related art is inapplicable.
FIG. 2 is a circuit diagram illustrating a configuration of a multiple output switching power source apparatus according to another related art. This multiple output switching power source apparatus consists of a first converter including a transformer T1, a switching element Q1, a rectifying-smoothing circuit (D1, C1), a feedback circuit 10-1, and a control circuit 11-1, a second converter including a transformer T2, a switching element Q2, a rectifying-smoothing circuit (D2, C2), a feedback circuit 10-2, and a control circuit 11-2, and a third converter including a transformer T3, a switching element Q3, a rectifying-smoothing circuit (D3, C3), a feedback circuit 10-3, and a control circuit 11-3. Namely, each output consists of a flyback converter. This circuit configuration can insulate outputs from one another.